Implement Polynomial Middle product

[bgrenet]

• Implement (a generalized version of) the Polynomial Middle Product as defined in Hanrot-Quercia-Zimmerman'04: If P has size m+n-1 and Q has size n, MP(P,Q) is the size-m polynomial made of the central coefficients of the product P×Q.
• Three implementations:
  • Standard quadratic algorithm : stdmidmul;
  • Karatsuba algorithm for the balanced case (m=n): karamidmul;
  • A dispatch algorithm for the general case that that either calls the quadratic algorithm for small sizes or Karatsuba's algorithm for larger sizes: midmul. (In particular, in the unbalanced case (m≠n), several calls are made to Karatsuba's algorithm with balanced inputs.)

The aim is to use this Middle Product in Newton iteration introduced in #206. This will be made in a future PR.

[jgdumas]

LGTM.